The recognized association of an inherited deficiency of adenosine deaminase (ADA) activity and an autosomal recessive form of severe combined immunodeficiency disease has stimulated interest in the genetic nature of the enzyme deficiency with respect to the pathogenesis of immune dysfunction. Our approach to a better understanding of the expression and regulation of this enzyme in normal, abnormal, and genetically deficient states will focus on: (1)\defining the primary structure of normal human ADA and the nature of its post-translational modification; (2)\characterizing the kinetic and physical properties of normal and mutant forms of ADA in cultured cells; and (3)\identifying the type and site(s) of mutation in ADA protein from patients with this enzyme deficiency. Current progress toward our objectives include cloning a partial ADA cDNA sequence that specifically cross-hybridizes with human chromosome 20 and obtaining 2/3 of the primary ADA amino acid sequence. In addition, the first acidic ADA mutation has been characterized from a subject with less than 1% lymphocyte enzyme activity but normal immune function. It is hoped that our goal-directed approach will allow a more precise definition of structural characteristics and processing of ADA protein in normal and genetically deficient patients. (LB)